Thermo-physical properties of reproducible Sip/Al composites

XIU Zi-yang(修子扬), WU Gao-hui(武高辉), SONG Mei-hui(宋美慧), ZHU De-zhi(朱德志)

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Three kinds of high volume fraction Si_p/1199, Si_p/4032 and Si_p/4019 composites were fabricated by squeeze casting method. The results show that the clean Si /Al interfaces without interfacial reaction products can decrease the interfacial thermal resistance. The composites have a low coefficient of thermal expansion (7.5×10^-6 ℃^-1) and high thermal conductivity ranging from 126 to 157.9 W/(m·℃). With increasing temperature, the specific capacity and the average coefficient of thermal expansion increases monotonically, the thermal diffusivity and the thermal conductivity decrease gradually. The specific capacity, average coefficient of thermal expansion, thermal diffusivity and the thermal conductivity of the composites decrease gradually with increasing Si content. The thermal conductivities of composites were calculated by theoretical models. Both Maxwell model and P.G model consider the reinforcement as nearly-round particles, and the interface thermal resistance of Si_p/Al composite calculated by EMA method is 0.01×10^-6 m²·℃/ W.

Key words: Si_p/Al composite; interfaces; thermal expansion; thermal conductivity

1 Introduction

Materials are the base of the industry, and advanced composites are the major in the current materials industry [1-5]. Especially, the advanced composites applied in aviation and aerospace are the harbinger and dominant in the application. But the sole metal material can not meet the demands of the aviation technology development [6-11]. Particulate reinforced aluminum matrix composites, which have low and designable thermal expansion property, excellent thermal conductivity and low density, have obtained extensive application in aviation field. The Si_p/Al composites developed newly were adopted as its low expansion (7.4-9.0×10^-6/℃), high thermal conductivity (100-180 W/(m·℃), low density (2.4-2.6 g/cm³), excellent machining property and ability to reclaim after melted, which attracts more attention in the development of metal matrix composites [12-15].

But the study on Si_p/Al composites just started, the preparation and basic properties measurement were the major, while further investigation is seldom reported. The matrix alloys used were 1199, 4032 and 4019 fabricated by squeeze-casting method, the effects of Si content on thermo-physical property were studied.

2 Experimental

The Si_p/1199 were annealed at 613 K for 3 h and furnace cooled. The other two composites were annealed at 673 K for 3 h and also furnace cooled. The thermal conductivity measurement was performed on JK2 Thermal Diffusivity Analysis made in Germany. The specimen size was d12.7 mm×3 mm, and both ends of which were polished with sand paper. The tested temperatures were from 298 to 773 K with a rate of 5 K/min. The CTEs of Si_p/Al composites were measured on a DIL 402C (NETZSCH Corp.) with a specimen size was d 6 mm×25 mm. The tested temperatures were from 298 to 773 K with a rate of 5 K/min, with a helium flux rate of 50 mm/min.

3 Results and discussion

Silicon is the main alloy element in the 4032 and 4019 alloys. When the reinforcement particle content is uniform, it can be considered that the silicon element exists in the form of Si particles and Si element within the pure aluminum matrix composite. The effects of gross silicon content on the thermal conductivity of composites can reflect the difference by the matrix alloy.  The formula for calculating the silicon content within the composite is shown as

               (1)

where  φ_fSi is the gross Si content; φ_f is Si particles content; r_m is the density of matrix alloy; r_Si is the density of Si; w_t is the quality fraction of Si in the matrix alloy.

The silicon content within the composite was calculated according to Eqn.(1) and the result is listed in Table 1.

Table 1 Total volume fraction of Si element for several Si_p/Al composite

3.1 Interface condition

The interface is an important factor that will affect the properties of composites. A representative TEM micrograph of the Si_p/Al composite interface is shown in Fig.1. It is clean and free from any interfacial reaction products.

Generally speaking, the following reactions occur around the interfaces:

4Al (l) +Al₂O₃(s) =3Al₂O (g)                    (2)

4Al (l) + SiO₂(s) = 2Al₂O₃(s) +3Si(s)              (3)

The rupturing of the Al₂O₃ films and the happening of interface reactions improves the united conditions of the Si/Al interfaces and decreases the resistance of interfaces. These are beneficial to the thermo-physical properties of materials.

3.2 Linear coefficient of thermal expansion

Fig.2 shows the linear CTEs of three Si_p/4032 composites at different temperatures. It can be seen that the Si_p/1199, Si_p/4032 and Si_p/4019 composites have a CTE value of 8.2×10^-6, 8.0×10^-6, and 7.5×10^-6 ℃^-1, respectively. The CTEs of composites increase with increasing temperature. For a metal matrix composite, its CTE mainly depends on the CTEs of matrix alloy and the interface’s influences where the particles pass. On one hand, the CTEs of composites will increase with increasing temperature. On the other hand, when the temperature increases, the load-transferring ability of interface decreases, leading to a decreased restriction against the expansion of the matrix, which leads to the linear increase of CTEs of composites with the temperature.

Fig.1 Interfaces in Si_p/Al composites

Fig.2 Effect of matrix alloy on CTE of composites

3.3 Specific heat capacity at constant pressure

Fig.3 shows the specific heat capacity of three Si_p/Al composites at constant pressure under different temperatures. It can be seen that the specific heat capacity at constant volume increases with increasing temperature. The known Debye law is shown as

                      (4)

where  c_v is specific heat capacity at constant volume；N is Avogadro constant; K is Boltzmann constant; T is absolute temperature; θ_D is Debye temperature. According to Eqn.(4), at a lower temperature, c_v is direct ratio to T ³.

Fig.3 Effect of matrix alloy on specific heat capacity of composites

When temperature is higher than Debye temperature, the c_v tends to the constant 3R. It can be seen from the thermodynamics theory that the relation between specific heat capacity at constant pressure and specific heat capacity at constant volume is shown as

                          (5)

where  a is bulk expansion coefficient; b is compressive coefficient at constant temperature; V is atom volume.

It can be seen from Eqn.(5) that the specific heat capacity at constant volume increases soon with temperature increase. This indicates that the thermal reservation capacity strengthens with temperature increase.

The specific heat capacity of composite independence to the matrix alloy is shown in Fig.3. It can be seen that the specific heat capacity of composite decreases steeply during constant temperature with silicon content of matrix increasing. From Table 1 it can be seen that with increasing silicon particles volume fraction, the silicon element content of matrix becomes higher, whereas the specific heat capacity of composite becomes lower.

3.4 Thermal diffusivity and thermal conductivity

The thermal conductivity λ of composite is calculated by

λ=αc_pρ                                      (6)

where  a is thermal diffusivity, ρ is density, c_p is specific heat capacity at constant pressure. Fig.4 shows the thermal diffusivity and thermal conductivity of three Si_p/Al composites under different temperatures. It can be seen that the thermal diffusivity and thermal conductivity of Si_p/1199 samples are the highest at constant volume fraction and temperature, whereas Si_p/4019 samples are the lowest. As shown in Table 1, the silicon element content of Si_p/4019 composite is the highest while that of Si_p/1199 composite is the lowest. The thermal conductivity of silicon is much higher than that of matrix (thermal diffusivity of Si was 89.2 mm²/s，its thermal conductivity is 149 W/(m·℃), the thermal diffusivity of LG is 589.9 mm²/s，its thermal conductivity is 235.2 W/(m·℃) [9]). Therefore, the increasing silicon content would lead to degrading thermal conductivity, and the silicon not only forms particles but also turns into matrix, it would introduce a lot of Si-Al interfaces and deteriorate the thermal conductivity. So, the thermal conductivity of composite would be decreased with increasing silicon content, but the particles volume fraction is settled. The increasing of reinforcement volume fraction can introduce more interfaces, which can hinder the thermal conduct greatly, so as to decrease the thermal conductivity of the composites.

Fig.4 Effect of matrix alloy on thermal conduction properties of Si_p/Al composites: (a) Thermal diffusivity; (b) Thermal conductivity

3.5 Calculated values of thermal conductivity and interface thermal resistance

The prominent characteristic of metal matrix composite was to be designable. And most of researchers were devoted to forecast the properties, or to control the components of materials with the properties demanded. Thus, the production periods and designation of novel materials can be shortened. And a perfect and exact theory foundation should be established firstly. The theoretical predictions model of PMMCs thermal conductivity was the Maxwell model and P.G model.

1) Maxwell

According to the conductance and the thermal conductivity property of biphasic and multiphase, the expression of the thermal conductivity is deduced as

            (7)

where  K refers to the thermal conductivity, φ_d refers to the volume fraction of the reinforcement，the foot-noted ‘com’, ‘m’ and ‘p’ refer to the composite, matrix and the particulate.

2) P. G. Klemens

The expression of the thermal conductivity of composite related to matrix, the thermal conductivity of reinforcement and its content was deduced by GKlemens as

    (8)

Table 2 lists the thermal conductivity calculated by the above models. By the introduction of reinforcement, a lot of interfaces are introduced into the composites. But the models mentioned above are not deemed the existing of the interface thermal resistance, so that values of test were less than those of calculated. The Si_p/Al composites interface thermal resistance was less. If the values fixed can be obtained by an effective method, it can stead to the design of materials in the future. And now, the experimental method cannot solve this problem. Therefore, the interface thermal resistance of Si_p/Al composite was calculated in this paper.

Table 2 Predicted and experimental thermal conductivities of Si_p/Al composite (W·m^-1·℃^-1)

The studies on interface thermal resistance of Si_p/Al composites were carried out by Hasselman and Johnson. They extended the Maxwell model and P.G model, and considered the effects of particles size on the interface thermal resistance. The effective medium approximation method was brought forward as:

 (9)

where  R_Bd is interface thermal resistance of composite,  d is particle diameter.

The formula of interface thermal resistance is deduced by the expression above, which is shown as

                      (10)                       

The interface thermal resistance value of Si_p/Al composite calculated by EMA method was 0.01×10^-6  m²·℃/W. But, the EMA method can not predict the effects of particles size on the interface thermal resistance completely, and more researches on this are needed.

4 Conclusions

1) Si_p/Al composite interface was clear and free from any interfacial reaction products, which is beneficial to the thermo-physical properties of materials.

2) The composites have a low coefficient of thermal expansion (7.5×10^-6 ℃^-1) and high thermal conductivity ranging from 126 to 157.9 W/(m·℃).

3) With increasing temperature, the specific heat capacity at constant pressure and the average coefficient of thermal expansion increase monotonically, the thermal diffusivity and the thermal conductivity decreasing gradually. The specific heat capacity at constant pressure, the average coefficient of thermal expansion, or the thermal diffusivity and thermal conductivity of the composites decrease gradually with the increasing of Si content.

4) The thermal conductivities of composites are calculated by theoretical models. Both Maxwell model and PG model consider the reinforcement as nearly round particles, and the effects of interfacial thermal resistance on thermal conduction are ignored. The calculated thermal conductivities are higher than measured values.

5) The interfacial thermal resistance has great effect on the thermal conduction of multiphase materials, and the interface thermal resistance of Si_p/Al composite calculated by EMA method is 0.01×10^-6 m²·℃/W.

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(Edited by LONG Huai-zhong)

Foundation item: Project(2003AA305110) supported by the Hi-tech Research and Development Program of China

Corresponding author: XIU Zi-yang; Tel: +86-451-86402373-4051; E-mail:xiuzy@hit.edu.cn